I have got 6500 miles on mine, 10 speed is really nice for a slush box auto. I would still like a 6 speed manual even if I lost a bit of mileage. But IIRC 8th is 1:1, and both 9th and 10th are overdrives. I get around 2.5-3 mpg more than the wife's truck with the 6 speed auto, about 46,000 on hers, 20.5 mpg vs 17.5-18 mpg. We are about 7 miles of highway from town, 2.5 miles from the house to the highway, 7 miles of 55, then about 2.5 miles to town at 45, and then what ever it is to the job. I usually average about 17 miles each way to work.
I do generally keep my foot out of what ever I am driving, despite going 5-10 mph over the speed limit. The 3.5l is a powerhouse. When I was going from a 7.3 diesel to the 4.9l gas, I missed the power and torque. My boss's truck has the 5.8L and it feels like a dog in comparison, but most of that is the 4 speed auto and heavier vehicle weight . I really like the power, even if I rarely use it, love the responsiveness, and it rides almost as nice as a Chevy.
I rolled it across the scales at the dump last Friday, three totes of tools 200-250lbs, half a tank of gas approximately 18 gallons, and it said 5800 pounds. I think that my Duramax scaled out at 1000-1200 lbs more with a similar load.
No problems with either, and she drove hers twice to Fairbanks this winter and we did nothing special for the harsh cold, it was fine. I am not sure I am sold on the aluminum body, but Ford sure seems to have the bugs worked out of this generation of F-150. If something were to happen that we needed to replace either one, it would be another F-150 3.5L 10 speed.